When high-speed large-capacity optical fiber communication systems are built, numerous optical devices are used. Such devices include devices that extract an optical signal of an arbitrary wavelength from an optical signal obtained by multiplexing a plurality of wavelengths, devices that use an optical crystal for matching the phase of the optical signal, and the like, and numerous optical fiber collimators which convert an optical signal emitted and spread from an optical fiber into parallel beams or which cause parallel beams to collect in an optical fiber.
The main function of such optical fiber collimators is to propagate parallel beams for a desired distance without attenuation. Low insertion loss and high return loss are generally desired.
In order to realize such low insertion loss and high return loss, methods are often used in which anti-reflective coatings are provided on the entire lens surface and the end surface of an optical fiber. Alternatively, the end surface of an optical fiber close to a lens is diagonally disposed in order to obtain a higher return loss such that the reflected beam is reflected to the outside from the optical fiber core.
The optical fiber collimator shown in FIG. 4 (see JP-A-2004-302453), for example, has been known as a conventional optical fiber collimator of this type in which the end surface of the optical fiber is diagonally disposed. The optical fiber collimator 101 shown in FIG. 4 comprises a partially spherical lens 102 that has beam-transmitting spherical surfaces 102a having the same radius of curvature at both ends of the cylindrical part, a tube 103 which contains an optical fiber 104 whose end surface 104a is inclined, and an eccentric sleeve 105 that has an passageway 105a for attaching the partially spherical lens 102 and tube 103. Furthermore, the central axis Z of the parallel beam emitted from the partially spherical lens 102 is within a radial range of 0.02 mm centered on the central axis B of the outer circumferential surface of the eccentric sleeve 105, and has an angle of 0.2° or less with respect to the central axis B of the outer circumferential surface of the eccentric sleeve 105.
With this optical fiber collimator 101, a high return loss can be obtained because the end surface 104a of the optical fiber 104 is inclined. Here, if the end surface 104a of the optical fiber 104 is inclined, the following problem arises: namely, a beam is emitted from the end surface 104a of the optical fiber 104 in a diagonal direction with respect to the central axis A of the partially spherical lens 102 in accordance with the law of refraction; as a result, in the parallel beam emitted from the partially spherical lens 102, an eccentricity δ is generated between the central axis Z of this parallel beam and the central axis A of the partially spherical lens 102. When an eccentricity δ is generated between the central axis Z of the parallel beam and the central axis A of the partially spherical lens 102, in cases where mutually facing optical fiber collimators are aligned with reference to the external diameter, a lack of alignment of the central axis Z of the parallel beams becomes a problem. However, in the case of the optical fiber collimator 101 shown in FIG. 4, because the central axis Z of the parallel beam emitted from the partially spherical lens 102 is within a radial range of 0.02 mm centered on the central axis B of the outer circumferential surface of the eccentric sleeve 105, and has an angle of 0.2° or less with respect to the central axis B of the outer circumferential surface of the eccentric sleeve 105, in cases where mutually facing optical fiber collimators 101 are aligned with reference to the external diameter, the central axes Z of the parallel beams substantially coincide.
However, it is difficult to set the optical axis Z of the parallel beam emitted from the partially spherical lens 102 within a radial range of 0.02 mm centered on the central axis B of the outer circumferential surface of the eccentric sleeve 105, and also within an angle of 0.2° with respect to the central axis B of the outer circumferential surface of the eccentric sleeve 105. Therefore, there is a problem in that the central axes Z of the parallel beams may not coincide in cases where mutually facing optical fiber collimators 101 are aligned with reference to the external diameter.
In contrast, the device shown in FIG. 5 (see the specification of U.S. Pat. No. 5,384,874), for example, has been known as an optical fiber rod lens device which realizes a low insertion loss and high return loss, and which eliminates the eccentricity of the central axis of parallel beam emitted from the lens with respect to the central axis of the lens. FIG. 5 is a diagram showing the basic construction of a conventional optical fiber rod lens device.
The optical fiber rod lens device 201 shown in FIG. 5 comprises an optical fiber 202 consisting of a core 202a and a cladding 202b surrounding the core 202a, and a convergent rod lens 203 connected to the end surface of the optical fiber 202. Furthermore, the optical fiber 202 and rod lens 203 are designed to be connected to each other by fusion such that the central axes of these parts are aligned with each other.
With this optical fiber rod lens device 201, because the optical fiber 202 and rod lens 203 are connected to each other by fusion such that the central axes of these parts are aligned with each other, it is possible to realize a low insertion loss and high return loss and to eliminate the eccentricity of the central axis of parallel beam emitted from the lens with respect to the central axis of the lens.
However, in this optical fiber rod lens device 201, because the optical fiber 202 and rod lens 203 are connected to each other by fusion, the need for a large-scale manufacturing apparatus such as a CO2 laser and arc discharge apparatus is a problem.
In contrast, the optical connector shown in FIGS. 6A and 6B (see JP-A-5-113519), for example, has been known as an optical connector which realizes a low insertion loss and high return loss, which eliminates the eccentricity of the central axis of parallel beam emitted from the lens with respect to the central axis of the lens, and which does not require any large-scale manufacturing apparatus. FIGS. 6A and 6B show a conventional optical connector; FIG. 6A is a sectional view, and FIG. 6B is an explanatory diagram of the optical connector in a use state.
The optical connector 301 shown in FIG. 6A comprises a connector main body 310, an optical fiber 320, and a spherical lens 330. The connector main body 310 is formed from an opaque resin or the like. The connector main body 310 is provided with a circular conic opening 311 that holds the lens 330, an optical fiber receiving opening 312 that is bored so that its central axis coincides with the central axis of the circular conic opening 311, and alignment openings 313 that are used during mating with a mating optical connector 301 (see FIG. 6B). Furthermore, the optical fiber 320 is inserted into the optical fiber receiving opening 312 from the opposite side of the circular conic opening 311, and is fastened in place by an adhesive. The fastening of the optical fiber 320 is accomplished so that the position of the end of the optical fiber 320 is at the focal point of the optical system that is determined by the diameter and refractive index of the lens 330 and the refractive index of a photocurable resin 340 (described later). Moreover, the silicone buffer 321 and jacket 322 of the optical fiber 320 are also bonded and fastened to the connector main body 310.
Meanwhile, a transparent photocurable resin 340 having substantially the same refractive index as those of the optical fiber 320 and lens 330 is injected into the circular conic opening 311, and the lens 330 is inserted on top of this so that this lens 330 contacts the wall of the circular conic opening 311, thus fastening this lens in place by photocuring of the photocurable resin.
As is shown in FIG. 6B, this optical connector 301 is positioned, abutted, and fastened to the mating connector 301 by the alignment openings 313 and guide pins 314. Furthermore, a beam emitted from the optical fiber 320 of one optical connector 301 passes through the transparent photocurable resin 340, is converted into parallel beams by the lens 330, enters the lens 330 of the other mating optical connector 301, is focused by this lens, further passes through the photocurable resin 340, and is caused to converge at the end surface of the optical fiber 320.
In this optical connector 301, because the optical fiber 320 and lens 330 are fastened by the transparent photocurable resin 340 having substantially the same refractive index as those of the optical fiber 320 and lens 330, a low insertion loss and high return loss can be realized. Moreover, the optical fiber insertion and fiber receiving opening 312 is bored so that the central axis of this fiber receiving opening 312 coincides with the central axis of the circular conic opening 311, and the optical axis of the optical fiber 320 coincides with the central axis of the spherical lens 330; therefore, it is possible to eliminate the eccentricity of the central axis of the parallel beam emitted from the lens 330 with respect to the central axis of the lens 330. In addition, because there is no need to connect the optical fiber 320 and lens 330 by fusion, a large-scale manufacturing apparatus such as an arc discharge apparatus is not required.
However, the following problems have been encountered in this conventional optical connector 301 shown in FIGS. 6A and 6B.
Specifically, the optical fiber 320 is fastened to the connector main body 310 so that the position of the end of the optical fiber 320 is the focal point of the optical system in this optical connector 301. However, there is no mechanism for positioning the optical fiber 320 in the direction of optical axis. Accordingly, when this optical fiber 320 is fastened to the connector main body 310, it is necessary to determine the position of the tip end of the optical fiber 320 while optically monitoring this optical fiber, so that there is a problem in that it is difficult to position the optical fiber in such a manner that the position of the end of the optical fiber 320 is the focal position of the optical system.
Furthermore, the photocurable resin 340 that fastens the lens 330 to the wall of the circular conic opening 311 is injected into the circular conic opening 311 and cured by photocuring after the lens 330 is inserted on top of this resin. Therefore, there is a danger that gas or foreign matter will be mixed in. If gas or foreign matter is mixed into the photocurable resin 340, there is a problem in that beam is scattered when passing though the photocurable resin 340, so that the transmitted beam is attenuated.
Moreover, because the optical fiber 320 is directly inserted into the fiber receiving opening 312, accidents occur in some cases such as breakage of the optical fiber 320 during handling.